Embedding data in an information signal

ABSTRACT

The present invention controls copying of an information signal, comprises the steps of prior to recording and/or transmission, applying to the information signal a substantially imperceptible modification representing copy control data including a password securely encoded according to a predetermined algorithm; upon reproduction for copying by a user, deriving (S 1 , S 3 ) the copy control data from the modified information signal; comparing (S 8 , S 9 , S 11 , S 13 , S 15 ) the derived securely encoded password with a reference password securely encoded according to a predetermined algorithm; and enabling (S 5 ) copying of the information signal if the securely encoded password derived from the information signal and the securely encoded reference password have a predetermined relationship, otherwise disabling copying (S 7 ). The reference password is sent to the user via a channel, which is separate from a channel used to send the information signal to the user.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to embedding data in an informationsignal. Examples of the invention relate to: a method of and system forcontrolling copying of an information signal; an information signal; adata carrier on which an information signal is recorded; apparatus formodifying an information signal; a reproducing apparatus; and a computerprogram.

2. Description of the Prior Art

U.S. Pat. No. 5,161,210 (US Philips Corporation) discloses a system forinhibiting copying of audio signals. An audio signal is divided intofrequency sub-bands and sub-band samples are quantized. The quantizedsamples are combined with samples of an auxiliary signal. The combinedaudio signal is recorded on a record carrier or transmitted. Theauxiliary signal is inaudible in the combined audio signal. An audiosignal reproducer having a recording unit also has a unit for detectingthe auxiliary signal and generating a record control signal. Therecording unit is constructed so that if a record control signal appearson its record control input the recording unit does not record the audiosignal.

WO 00/51348 (Macrovision) discloses a method and apparatus forinhibiting copying of audio or video signals transmitted over a cabletelevision or direct satellite broadcast or the Internet. The signal isprotected from unwanted copying by the combination of a watermarkembedded in the signal at the head end together with additional copyprotection data inserted in the signal. The additional data is a ticket.If the consumer pays a fee, the signal is transmitted to the consumer.The consumer can record only if the watermark and a mathematicalfunction of the ticket match. The function is for example a hashfunction.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of controlling copying of an information signal in a systemhaving a source of the information signal and a device for copying theinformation signal, the method comprising the steps of:

at an information signal modification source, prior to transmission ofthe information signal to the copying device, generating a copy controlpassword and a related reference password, and applying to theinformation signal a substantially imperceptible modificationrepresenting copy control data including the copy control passwordsecurely encoded according to a predetermined algorithm;

delivering the modified information signal from the modification sourceto the copying device via a communications channel;

delivering the reference password from the modification source to thecopying device via a separate communications channel independent of themodified information signal;

upon reception of the modified signal deriving the copy control datafrom the modified information signal;

comparing the derived securely encoded password with the separatelyprovided reference password securely encoded according to apredetermined algorithm; and

enabling copying of the information signal if the securely encodedpassword derived from the information signal and the securely encodedreference password have a predetermined relationship.

Thus the present invention provides for conditional control of recordingof an information signal instead of, or in addition to, simple denial ofany recording and simple complete freedom to record or otherwise copy.Recording may be any form of storage including but not limited tostorage on a linear data carrier, for example magnetic tape.

Furthermore, the reference password is not in the information signal oron any carrier of the information signal but is provided to the copyingdevice independently of the information signal.

The information signal may represent any one of, or a combination of,image information (including still and moving images), audio, video,text, and data which may be executable or otherwise.

In examples of the invention the said copy control data includes otherdata indicating that copying is permitted subject to the provision by auser of a correct reference password.

The reference password is preferably securely encoded according to thesame algorithm as the password derived from the information signal. Thesaid predetermined algorithm may be an encryption algorithm. Preferablythe said algorithm is a hash function in which case the referencepassword and the password derived from the information signal are thesame. The reference password may be delivered to the copying device as aplain password or securely encoded, e.g. encrypted in which case it isdecrypted at the copying device.

The reference password may be provided via a secure communicationschannel which is separate from the transmission channel of theinformation signal.

In an example of the invention, the reference password is provided to auser who wishes to copy the information signal. The password may beprovided on a secure data carrier, e.g. a smart card or in some other,preferably secure, way. If provided on a smart card the referencepassword can be kept secure even from the user. The user provides thepassword to the copying device via an input device , e.g. a keyboard ora card reader. The user may be prompted to provide the password.Preferably that is done in response to the copy control data whichindicates that copying is conditional upon the provision of thepassword. Thus the user is required to take positive action if they wishto copy an information signal for which copying is conditionallyallowed.

A second aspect of the invention provides a method of applying copycontrol data to an information signal comprising the steps of:

determining whether copying of the information signal is allowed, notallowed or conditionally allowed; and

applying to the signal a substantially imperceptible modificationrepresenting copy control data, the copy control data comprising

a) first data if copying is allowed,

b) second data if copying is not allowed, and

c) third data if copying is conditionally allowed,

the third data including at least a password securely encoded accordingto a predetermined algorithm.

A third aspect of the invention provides a method of controlling theoperation of a signal copying device having a recording unit controlledby a processor, the copying device being operable to record aninformation signal produced by the method of the second aspect, themethod comprising the steps of:

using the processor to derive the copy control data from the informationsignal and to determine whether the control data is the first, second orthird data and to

a) allow the recording unit to record if the first data is present inthe information signal,

b) disable the record unit if the second data is present in theinformation signal; and

c) allow the recording unit to record if the third data is present inthe information signal and a reference password is provided which whensecurely encoded by a predetermined algorithm has a predeterminedrelationship to the said securely encoded password of the third data.

The second and third aspects of the present invention provide copycontrol data which explicitly indicates, and allows, conditional controlof recording of an information signal and, in addition, simple denial ofany recording and simple complete freedom to record.

It is possible that the copying device receives an information signalwhich does not have an imperceptible modification representing copycontrol data. In that case the copying device may be arranged to allowcopying (or in the alternative not allow copying).

The copy control data may be used to control the copying device tooperate in a predetermined manner. For example it may control the formof any copies for example indicating the form of copy control data to beincluded in any copy.

These and other aspects of the invention are set out in the accompanyingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill be apparent from the following detailed description of illustrativeembodiments which is to be read in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic block diagram of an illustrative apparatus forapplying a modification to an information signal in accordance with theinvention;

FIG. 2 is a schematic block diagram of an illustrative signalreproducing and recording apparatus in accordance with the invention;

FIGS. 3A and B are flow diagrams illustrating an operation performed bya processor of the apparatus of FIG. 2;

FIG. 4 is a schematic block diagram of an illustrative system forcontrolling copying in accordance with the invention;

FIG. 5 is a schematic block diagram of an illustrative watermarkingapparatus useful in the apparatus of FIG. 1; and

FIG. 6 is a schematic block diagram of an illustrative apparatus usefulin the apparatus of FIG. 2 for detecting a watermark and extracting datatherefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a signal modifying apparatus 30 comprises a source2 which produces an information signal which may be for example an audiosignal, a video signal, an audio/video signal, a data signal and/or animage signals. The source 2 may be any suitable source for example asignal reproducer which reproduces the signal from a record or anoriginal source for example a camera in the case of video or microphonein the case of audio.

For ease of explanation the following description assumes theinformation signal is a video signal, but the invention is not limitedto video.

A watermarking apparatus 6 receives the video signal from the source 2and applies to it copy control data from a source 4. In this example thewatermarking apparatus 6 embeds the copy control data in the video insuch a way that the embedded data is substantially imperceptible in thevideo. Watermarking techniques are known in the art of video and anexample of such a technique is described below with reference to FIG. 5but any other suitable known watermarking technique may be used.

The watermarking apparatus may optionally embed as a watermarkprovenance data, metadata, or other data in addition to the copy controldata.

The copy control data is embedded for the purpose of controlling copyingof the video signal. In one example of the invention, the data comprisesa code h which is a hash function H(p) of a password p: i.e. the codeh=H(p). In another example of the invention, the control data h isassociated with further data, for example 01 or 10, which indicates thepresence of the code h.

In an example of the invention, the copy control data comprises aselected one of the following copy status codes:

00 which indicates no copying is allowed;

11 which indicates copying is freely allowed; and

01 or 10 together with code h which indicates that copying is allowedprovided a reference password is provided which when hashed by the hashfunction H matches the code h.

The codes 00, 11 and 10 or 01 are examples of codes for simplicity ofexplanation. More complex codes may be used, Preferably codes which areunlikely to occur by chance are used.

The following description describes the currently preferred example butthe invention is not limited to that example.

The video into which the control data is embedded by the watermarkingapparatus 6 is fed to a recorder or transmitter 8. If fed to a recorder,the recorder 8 may record the watermarked video on a data carrier forexample a disc or tape or semiconductor memory. If transmitted, thetransmitter 8 may be for example a broadcast apparatus or a server whichtransmits the watermarked video to a distribution system.

Reference numeral 10 indicates a schematic representation of adistribution system which may be amongst other examples: an electroniccommunications network 15 for transmitting the video e.g. a broadcastnetwork, a PSTN or the Internet; or a physical distribution network viawhich tapes 13 or discs 11 or other data carriers on which the video isrecorded are distributed.

The code source 4 also provides a reference password which in thisexample is the password p. The reference password p is fed via aninterface 5 to a secure data carrier, e.g. a smart card SC for deliveryto a user of the video separately from the video. It will be appreciatedthat the reference password could be delivered by other means, e.g.another form of data carrier, on paper through the post or via theInternet or telephone system.

FIG. 2 illustrates a copying device which in this example is areproducing and recording apparatus 32. The apparatus 32 comprises asource 12 of video which may or may not be watermarked. The source 12may reproduce the video from a data carrier or receive the video from abroadcast or other communications system as described above. The videois applied to a recording unit 22. The recording unit 22 is controlledby a record control signal produced by a control processor 14.

Assume the video is watermarked.

The control processor 14 comprises a watermark processor 16 whichdetects the watermark and derives the copy control data therefrom.Watermark processors capable of doing that are known and an example isdescribed with reference to FIG. 6 below. A processor 18 decodes thecopy control data and applies the appropriate record control signal tothe recording unit. The control processor also has an input device 20for entering the reference password. The input device may be a keyboard,smart card reader, an interface with an electronic communicationschannel, amongst other examples. In addition a display 17 may beprovided. The display is used in an example of the invention to promptthe user to enter the reference password via the input device, forexample by inserting the smart card SC into the input device 20.

Referring to FIGS. 3A and B, the control processor 14 operates asfollows:

In step S1 the watermark processor 16 detects the watermark and derivesthe copy control data. In step S3 the processor 18 determines the valueof the copy control data.

If the copy control data is:

00 then the record control signal is set S7 to disable the recordingunit;

11 then the record control signal is set S5 to enable recording by therecording unit; and

01 or 10 together with a code of any value then the user is requested S8to provide a reference password p′.

Referring to FIG. 3B, in step S81, the processor 18 detects whether thecopy control data includes the code 01 or 10 indicating copying isallowed if the reference password is provided. If so, it causes a promptto be displayed S82 by the display 17 requiring the user to enter thepassword.

If a reference password p′ is entered S9 via an input device 20 theprocessor performs S11A hash function H(p′) on the password and comparesS13 the hashed reference password with the hash value h derived from thewatermarked video. If the hashed values are the same S15 then copying isallowed S5. If the hashed values are not the same S15 then the recordingunit is disabled S7.

If recording is enabled because the reference password is correct orbecause the copy control data=11, then the watermarked video includingthe original copy control data is recorded to provide copy control ofthe copy.

The user of the reproducing apparatus 32 requires the reference passwordto copy video which has as copy control data the code 01 or 10 plus thehash value h. As will be described with reference to FIG. 4, thereference password is supplied to the apparatus 32 separately from andindependently of the watermarked video. In this example the password isprovided on the smart card SC which is read by a card reader 20 which isthe input device 20. This enables the password to be provided to a userand kept secret even from the user.

The smart card may be provided to a user on payment by the user of afee.

The video form the source 12 may not have a copy control watermark. Ifthe watermark processor 16 detects the absence of such a watermark, itindicates that to the processor 18; see step S1 of FIG. 3A). The absenceof the watermark may be regarded as either indicating copying is notallowed so that the processor outputs code 00 to processor 18. Currentlyit is preferred that the absence of the watermark indicates copying isallowed. Thus the processor 16 outputs code 11 to the processor 18.

The system of FIGS. 1 and 2 is illustrated by way of example as aspecial purpose system but could be implemented using programmable dataprocessors.

In the system of FIG. 4, the modifying apparatus 30 and the reproducingapparatus 32 are linked by communications interfaces I/F to acommunications network 38 which is for example the Internet. There maybe many reproducing apparatus 321 to 32 n connected to the network 38.For simplicity of description, it is assumed that the apparatus 30 iscontrolled by a seller and the user of the reproducing apparatus 32 is abuyer. Watermarked video is sent to buyers for example on a disc D via adistribution channel 10 for example a postal service or shop. If a buyerwishes to copy the watermarked video and it is watermarked with the copycontrol code 01 or 10 plus the hash value h, then the copy function inthe reproducing apparatus 32 is disabled until the buyer pays a fee. Thefee may be paid via the network 38 and a server 34, which for thatpurpose is linked to a financial institution 36, e.g. a credit cardcompany.

The copy control hash values and/or the password may be unique to eachuser for increased security and traceability of unauthorised copies.

The payment of the fee is communicated to the seller via the network 38.The seller then releases the reference password for example on a smartcard SC or in a secure manner via the communications network 38.Alternatively, the server may release the reference password on a smartcard or via the network 38. For that purpose the server may communicatewith the seller to obtain the reference password. It will be appreciatedthat the reference password could be delivered by other means, e.g. on adata carrier, on paper through the post or via the Internet or telephonesystem. Preferably the reference password is encrypted beforetransmission to the copying device 32 in which case it is decrypted atthe copying device. Any known encryption system may be used.

The present invention assumes that all reproducing and recordingapparatus are, equipped with watermark detection and decoding apparatuswhich disables the recording unit of the apparatus.

Watermarking, FIG. 5

FIG. 5 illustrates the watermarking apparatus denoted as embedder 120 inmore detail. The watermark embedder 120 comprises pseudo-random sequencegenerator 220, an error correction coding generator 200, a wavelettransformer 210, an inverse wavelet transformer 250, a first combiner230, a data converter 225 and a second combiner 240. The wavelettransformer 210 includes a frame store FS1. The inverse transformer 250includes a frame store FS2. The frame store FS1 stores a frame ofunmodified coefficients Ci. The frame store FS2 stores a frame ofmodified coefficients Ci′.

The error correction coding generator 200 receives the copy control dataand outputs an error correction coded copy control data to the firstcombiner 230. The pseudo-random sequence generator 220 outputs apseudo-random binary sequence (PRBS) Pi, where i is the i^(th) bit ofthe sequence, to the first combiner 230. The PRBS has a length L×J ofbits where J is the number of bits in the error correction encoded copycontrol data. Each bit j of the error correction encoded copy controldata then modulates a section of length L of the PRBS. The firstcombiner 230 logically combines the error correction encoded copycontrol data with the PRBS to produce a watermark having bits Ri. A bitWj=0 of the error correction encoded copy control data inverts L bits ofthe PRBS. A bit Wj=1 of the error correction encoded copy control datadoes not invert the PRBS. Thus bits Wj of the error correction encodedcopy control data are spread over L bits of the PRBS. The data converter225 converts binary 1 to symbol +1 and binary 0 to symbol −1 to ensurethat binary 0 bits contribute to a correlation value used in the decoderof FIG. 5.

The wavelet transformer 210 receives the video image I from the source110 and outputs wavelet coefficients Ci to the second combiner 240.

The second combiner 240 receives the watermark Ri, the waveletcoefficients Ci and watermark strength αi and outputs modifiedcoefficients Ci′ whereCi′=Ci+αi Ri

The inverse wavelet transformer 250 receives the modified coefficientsCi′ and outputs a spatial domain watermarked image I′.

The embedder includes an ECC generator 200. The use of error correctioncoding to produce an error correction coded copy control data isadvantageous since it allows the copy control data 175 to bereconstructed more readily should some information be lost. Thisprovides a degree of robustness to future processing or attacks againstthe watermark. The use of a pseudo-random sequence Pi to generate aspread spectrum signal for use as a watermark is advantageous since itallows the error correction coded copy control data 205 to be spreadacross a large number of bits. Also, it allows the watermark to be moreeffectively hidden and reduces the visibility of the watermark. Applyingthe watermark to a wavelet transform of the image is advantageous sincethis reduces the perceptibility of the watermark. Furthermore, thestrength of the watermark is adjusted by αi to ensure that the watermarkis not perceptible.

Detecting Copy Control Data in Watermarked Video, FIG. 6

The operation of the watermark processor denoted as decoder 140 will nowbe explained in more detail with reference to FIG. 6. The watermarkdecoder 140 receives the watermarked image I′ and outputs the restoredcopy control data. The watermark decoder 140 comprises a wavelettransformer 310, a reference pseudo-random sequence (PRBS) generator320, a correlator 330, a selector 340 and a error correction codingdecoder 350. The PRBS generated by the generator 320 is identical tothat generated by the PRBS generator 220 of FIG. 2 and converted by adata converter (not shown) to values +1 and −1 as described above.

The wavelet transformer 310 receives the watermarked image I′ and, inknown manner, outputs the modified wavelet coefficients Ci′. Thecorrelator 330 receives the reference pseudo-random sequence PRBS havingsymbols Pi of values +1 and −1 from the pseudo-random sequence generator320, and the wavelet coefficients Ci′ and outputs a watermark image bitcorrelation sequence 335. The watermarked image bit correlation sequenceis determined in the following way.

The modified wavelet coefficients Ci′=Ci+α_(i)R_(i) where R_(i) are bitsof PRBS modulated by error-correction encoded bits Wj of copy controldata. Each bit Wj modulates L bits of PRBS. There are JL bits in themodulated PRBS. For each error correction encoded bit Wj, the correlator330 calculates a correlation value

$S_{j}^{\prime} = {\sum\limits_{i = {{jL} + 1}}^{{jL} + L}{{Ci}^{\prime} \cdot {Pi}}}$where j=0, 1, 2 . . . J−1, and J is the number of error correctionencoded bits. A sequence 335 of correlation values S′_(j) is produced.

The correlation sequence 335 is received by the selector 340 whichoutputs an uncorrected copy control data 345. The selector 340 outputs abit value “1” for a value of S′ greater than 0 and a bit value “0” forS′ less than or equal to 0. The error correction code decoder 350receives the uncorrected copy control data 345 and in known manneroutputs the restored copy control data 145.

The reference PRBS Pi is synchronised with the modulated PRBS in thewatermarked image. For that purpose a synchroniser (not shown) is used.Such synchronisation is known in the art.

Modifications

Although FIGS. 5 and 6 give an example of watermarking using Waveletcoefficients, the invention is not limited to Wavelets but can beimplemented using other watermarking techniques including the use of DCTcoefficients.

Although the invention has been illustrated by reference to FIGS. 1 to 4which show schematics of special purpose hardware, it is envisaged thatthe invention may be implemented in software on programmable machines.Thus the invention also encompasses software which when run on suitabledata processing equipment implements the functions described herein.

The information signal whether stored on a data carrier or sent as asignal via a communications channel may include several partsrepresenting different sections of content or different items ofcontent. For example a disc or tape typically has several tracks. In anembodiment of the invention each section may have its own copy controldata embedded as an imperceptible watermark each with its own copystatus and/or its own password. For example if a disc has tracks one tofour the copy control data may be as follows:

Track Copy Status Data Password Track 1 00 — Track 2 01 nnnnnn Track 301 mmmm Track 4 11 —Thus track 1 can be copied freely. Track 2 may be copied if the passwordnnnnn is provided. Track 3 may be copied if password mmmmm is provided,Copying of track five is not allowed. A user may be provided with onlyone of the passwords.

In a further embodiment, the copy status codes define copying rights fora user in addition to copying allowed, not allowed and conditionallyallowed. For example code 01(plus the password) may indicate copying isconditionally allowed but the copies retain the original copy controldata signifying the conditional copying status whereas code 10 mayindicate copying is conditionally allowed and the copy may be freelycopied, the copy control data not being retained in the first copy. Code10 may be used to protect content whilst in transit between a supplierand the user for example.

Other status codes with passwords forming the copy control data mayindicate other copying status.

In embodiments in which the copy status data is changed, the watermarkmust be amendable. Watermarks which may be removed are described in forexample co-pending European patent application 1215880. The processors16 and 18 in the control processor 14 of FIG. 2 are arranged to removethe original watermark and replace it with a new one. The new one may bepermanent or removable.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various changes and modifications can be effectedtherein by one skilled in the art without departing from the scope andspirit of the invention as defined by the appended claims.

1. A method of controlling copying of an information signal in a systemhaving a source of the information signal and a device for copying theinformation signal, the method comprising the steps of: at aninformation signal modification source, prior to transmission of theinformation signal to the copying device, generating a copy controlpassword and a related reference password, and applying to theinformation signal a substantially imperceptible modificationrepresenting copy control data including the copy control passwordsecurely encoded according to a predetermined algorithm; delivering themodified information signal from the modification source to the copyingdevice via a communications channel; delivering the reference passwordfrom the modification source to the copying device via a separatecommunications channel independent of the modified information signal;upon reception of the modified signal deriving the copy control datafrom the modified information signal; comparing the derived securelyencoded password with the separately provided reference passwordsecurely encoded according to a predetermined algorithm; and enablingcopying of the information signal if the securely encoded passwordderived from the information signal and the securely encoded referencepassword have a predetermined relationship.
 2. A method according toclaim 1, wherein the reference password is securely encoded according tothe same algorithm as the derived password.
 3. A method according toclaim 1, wherein the copy control data additionally includes other dataindicating that copying is permitted if a correct reference password isprovided.
 4. A method according to claim 1, wherein the copy controlpassword and the reference password are the same and the predeterminedalgorithm is a hash function.
 5. A method according to claim 1, whereinthe reference password is delivered to the copying device by the stepsof providing the reference password to a user of the information signalwho provides the reference password to the copying device.
 6. A methodaccording to claim 5, wherein the reference password is provided to theuser if the user fulfils at least one predetermined condition.
 7. Amethod according to claim 6, wherein the condition is payment forprovision of the password.
 8. A method according to claim 5, wherein thepassword is provided to the user via a secure communications channel. 9.A method according to claim 5, wherein the password is provided to theuser on a secure data carrier.
 10. A method according to claim 1,comprising the step of prompting the user to provide the password, theprompting of the user occurring in response to the derivation of thecopy control data.
 11. A method according to claim 1, comprising thestep of storing the modified information signal on a data carrier forsupply to the said user.
 12. A method according to claim 1, wherein themodification is applied to the information signal by modifying transformcoefficients of the information signal.
 13. A method according to claim12, wherein the said coefficients are DCT or Wavelet coefficients.
 14. Amethod according to claim 1, wherein the information signal comprisesany one or more of: image information, video information, audioinformation, text and data.
 15. A system for controlling copying of aninformation signal the system having a source of the information signaland a device for copying the information signal and further comprising:an information signal modification apparatus operable, prior totransmission of the information signal to the copying device, togenerate a copy control password and a related reference password, andapply to the information signal a substantially imperceptiblemodification representing copy control data including the copy controlpassword securely encoded according to a predetermined algorithm; acommunications channel for delivering the modified information signalfrom the modification source to the copying device; and a separatecommunications channel independent of the modified information signalfor delivering the reference password from the modification source tothe i copying device; wherein the copying device comprises a dataprocessor and an input device for providing the reference password tothe data processor, the data processor being operable to a) derive thecopy control data from the modified information signal upon reception ofthe modified signal, and b) compare the derived securely encodedpassword with the separately provided reference password securelyencoded according to a predetermined algorithm; and c) a copying unitenabled by the data processor to copy the information signal if thesecurely encoded password derived from the information signal and thesecurely encoded reference password have a predetermined relationship.16. A system according to claim 15, wherein the data processor isarranged to securely encode the reference password according to the samealgorithm as the derived password.
 17. A system according to claim 15,wherein the predetermined algorithm is a hash function.
 18. A systemaccording to claim 15, wherein the input device is a keyboard.
 19. Asystem according to claim 15, wherein the input device is a reader forreading data from a data carrier.
 20. A system according to claim 15,wherein the input device comprises a communications interface forreceiving the reference password from a communications network.
 21. Asystem according to claim 15, wherein the signal modification apparatusis operable to apply the modification by modifying transformcoefficients of the information signal.
 22. A system according to claim21, wherein the transform coefficients are DCT or Wavelet coefficients.23. A system according to claim 15, further comprising means forprompting a user to provide the reference password to the data processorvia the input device.
 24. Processing apparatus for copying aninformation signal to which is applied a substantially imperceptiblemodification representing copy control data including a passwordsecurely encoded according to a predetermined algorithm, the copyingdevice comprising: a data processor operable to receive the modifiedinformation signal, and an input device for receiving a referencepassword separately from, and independently of, the modified informationsignal, and for providing the reference password to the data processor,the data processor being operable to a) derive the copy control datafrom the modified information signal upon reception of the modifiedsignal, and b) compare the derived securely encoded password with theseparately provided reference password securely encoded according to apredetermined algorithm; and c) a copying unit enabled by the dataprocessor to copy the information signal if the securely encodedpassword derived from the information signal and the securely encodedreference password have a predetermined relationship.
 25. A processingapparatus according to claim 24, wherein the data processor is arrangedto securely encode the reference password according to the samealgorithm as the derived password.
 26. A processing apparatus accordingto claim 24, wherein the predetermined algorithm is a hash function. 27.A processing apparatus according to claim 24, wherein the input deviceis a keyboard.
 28. A processing apparatus according to claim 24, whereinthe input device is a reader for reading data from a data carrier.
 29. Aprocessing apparatus according to claim 24, wherein the input devicecomprises a communications interface for receiving the referencepassword from a communications network.
 30. A processing apparatusaccording to claim 24, further comprising means for prompting a user toprovide the reference password via the input device.
 31. A computerprogram which when run on a suitable processing system implements themethod of claim 1.